The most popular loudspeaker reproduction system is based on two-channel stereophony, using two loudspeakers at predetermined positions. If a user is located in a sweet spot, a technique referred to as amplitude panning positions a phantom sound source between the two loudspeakers. The area of feasible phantom source is however quite limited. Basically, phantom source can only be positioned at a line between the two loudspeakers. The angle between the two loudspeakers has an upper limit of about 60 degrees, as indicated in S. P. Lipshitz, “Stereo microphone techniques; are the purists wrong?”, J. Audio Eng. Soc., 34:716-744, 1986. Hence the resulting frontal image is limited in terms of width. Furthermore, in order amplitude panning to work correctly, the position of a listener is very restricted. The sweet spot is usually quite small, especially in a left-right direction. As soon as the listener moves outside the sweet spot, panning techniques fail and audio sources are perceived at the position of the closest loudspeaker, see H. A. M. Clark, G. F. Dutton, and P. B. Vanderlyn, “The ‘Stereosonic’ recording and reproduction system: A two-channel systems for domestic tape records”, J. Audio Engineering Society, 6:102-117, 1958. Moreover, the above reproduction systems restrict an orientation of the listener. If due to head or body rotations both speakers are not positioned symmetrically on both sides of a midsaggital plane the perceived position of phantom sources is wrong or becomes ambiguous, see G. Theile and G. Plenge, “Localization of lateral phantom sources”, J. Audio Engineering Society, 25:196-200, 1977. Yet another disadvantage of the known loudspeaker reproduction system is that a spectral coloration that is induced by amplitude panning is introduced. Due to different path-length differences to both ears and the resulting comb-filter effects, phantom sources may suffer from pronounced spectral modifications compared to a real sound source at the desired position, as discussed in V. Pulkki and V. Karjalainen, M. and Valimaki, “Coloration, and Enhancement of Amplitude-Panned Virtual Sources”, in Proc. 16th AES Conference, 1999. Another disadvantage of amplitude panning is the fact that the sound source localization cues resulting from a phantom sound source are only a crude approximation of the localization cues that would correspond to a sound source at the desired position, especially in the mid and high frequency range.
Compared to loudspeaker playback, stereo audio content reproduced over headphones is perceived inside the head. The absence of an effect of the acoustical path from a certain sound source to the ears causes the spatial image to sound unnatural. The headphone audio reproduction that uses a fixed set of virtual speakers to overcome the absence of the acoustical path suffers from drawbacks that are inherently introduced by a set of fixed loudspeakers as in loudspeaker playback systems discussed above. One of the drawbacks is that localization cues are crude approximation of actual localization cues of a sound source at a desired position, which results in a degraded spatial image. Another drawback is that amplitude panning only works in a left-right direction, and not in any other direction.